Sunflower downy mildew, caused by Plasmopara
halstedii, can be an economic problem in North Dakota, where
approximately half of the United States sunflower crop is grown. Prevalence and incidence
of downy mildew in North Dakota was assessed in mid-season and late-season surveys
annually from 2001 to 2011. In aggregate, 2,772 fields were evaluated. Downy
mildew levels fluctuated annually, and no clear connection between statewide
rainfall or management tools available to growers could be made. Surveys at
mid season enabled a two-fold higher detection in prevalence and incidence of downy
mildew than in late-season surveys due to the disappearance of early infected
plants by end of season. Both surveys identified years with relatively higher
levels of downy mildew but were inconsistent when lower levels of the pathogen
were recorded. The results of this study suggest that mid-season assessment of
downy mildew more accurately estimates disease pressure than late-season
assessment.

Introduction

Sunflower (Helianthus spp.) is native to North
America, but cultivated sunflowers (Helianthus annuus) only emerged as a
modern field crop in the United States in the 1970s (19). Approximately
one million hectares of sunflowers are planted annually in the US (18).
Production is concentrated in the Great Plains states of North Dakota,
South Dakota, Minnesota, Nebraska, Colorado, Kansas, and Texas, with North
Dakota producing
approximately 50% of the crop (2,18). Approximately 80% of the sunflowers
planted in North Dakota are oilseed hybrids, while the remainder are consumed as seed
(confection) (2).

Downy mildew is an economically important disease,
caused by Plasmopara halstedii (Farl.) Berl and de Toni, an obligate
Oomycete pathogen, which is indigenous to North America and can infect H.annuus and other Helianthus species (14). Primary infection occurs
when zoospores of P. halstedii infect seedling roots, causing a systemic
infection with signs and symptoms that include systemic chlorosis and prolific
production of zoosporangia on leaves (Fig. 1), severe stunting (Fig. 2), post
emergence damping-off, and horizontal and unproductive heads on surviving plants
(4,10). Water-saturated soils soon after planting favor systemic infection by
providing the environment for motile zoospores to reach seedling roots. As such,
low-lying areas in fields where water collects can have a high incidence of
infected plants and result in fields with large bare spots and/or areas with
little to no yield (4,10). Secondary infection by airborne sporangia can cause
discrete lesions on foliage (Fig. 3), but they are rarely of economic concern.

Fig. 1. Plasmopara halstedii sporulation on the underside of an infected sunflower leaf.

Management of downy mildew relies mainly on genetic
resistance and fungicidal seed treatments (10). However, the variability of P.
halstedii has repeatedly challenged management options. Plasmopara
halstedii isolates resistant to FRAC 4 fungicides (metalaxyl and mefenoxam)
were identified in the late 1990s and found to be widespread and predominant in
North Dakota by the 2000s (8). Currently, only FRAC 11 seed-applied fungicides
azoxystrobin (Dynasty, Syngenta Crop Protection, Greensboro, NC) and fenamidone
(Idol, Bayer CropScience, Research Triangle Park, NC), are used in the US to
suppress/manage downy mildew, but are considered at high risk for fungicide
resistance development (17). Similarly, races of the pathogen that are virulent
on the most widely used resistance genes available in commercial hybrids (Pl6
and Pl7) were identified in 2009 (7).

Since the early 2000s, prevalence and incidence of
downy mildew in North Dakota sunflower production were monitored as part of two different
survey efforts, namely: (i) the North Dakota State University-Integrated Pest
Management (NDSU-IPM) survey, a mid-season survey whose objective is to monitor
diseases and insect pests in three to five field crops (16); and (ii) the National
Sunflower Association Crop Survey, a late-season survey with the objective of
collecting yield and agronomic data and identifying which insect, weed, and
disease problems affect production (13). The objectives of this study are to
report on the prevalence and incidence of downy mildew in North Dakota between 2001 and
2011, compare results of the two surveys and discuss factors that may have
influenced the occurrence of downy mildew in the past and may have effects in
the future.

Mid-season Survey Methods

The Cooperative Extension Service at North Dakota
State University (NDSU) coordinates a formal and annual survey (hereafter,
"mid-season survey") of insects and diseases of field crops between late May and
mid August (12,16). The survey is conducted by five to six personnel, made up
primarily of junior and senior undergraduate students majoring in an
agricultural discipline who are trained to identify and report disease incidence
and severity and insect density in five crops, namely: barley, canola, soybean,
sunflower, and wheat. Surveyors are located at four Research Extension Centers
around the state and the NDSU main campus at Fargo, and survey eight to twelve
specific counties in their regions; all North Dakota counties are surveyed. In general,
surveyors evaluate fields in one county each day of the week, visiting as many
as 12 fields per day, and surveying every county in a 2-week time period. The
number of fields per crop surveyed in each county is roughly proportional to hectareage, but fields chosen within each county are arbitrarily selected. For
example, in a county planted to 50% wheat, 20% barley, and 10% each sunflower,
soybean, and canola: five, two, one, one, and one field(s) of each crop would be
arbitrarily selected and surveyed. Approximately 1,800 to 2,000 fields are
annually surveyed, of which 5-10% are sunflower fields. The incidence of plants
exhibiting signs and/or symptoms of downy mildew is recorded by examining 40
plants in two-row pairs at five points along a W pattern for a total of 200
plants per field.

Late-season Survey Methods

The National Sunflower Association sponsors an annual
survey of sunflower fields across major production areas in the US at the end
of the growing season, typically mid September to mid October (hereafter,
"late-season survey") (3,11,13). The survey is coordinated by the NDSU
Cooperative Extension Service and is conducted by volunteer professionals from
universities, seed and chemical companies, crop consultants, and other
knowledgeable individuals. Prior to surveying, volunteers attend an all-day
training event, which includes weed, insect, and disease identification and
logistic instruction. Survey teams of two to four volunteers are created by
maximizing the volunteers’ diversity of expertise. In North Dakota, 12 to 18 survey teams
survey sunflower fields during the last two weeks of September.

The number of sunflower fields surveyed in each county
is proportional to the previous years sunflower hectacreage in that county as
estimated by the USDA. Initially, one field/2,000 ha in a county was surveyed,
but was later decreased to one field/5,000 ha. Within each county, sunflower
fields are arbitrarily selected, although a minimum distance of 3 km between
each field is encouraged. Two locations representative of the field are
selected. The incidence of sunflowers infected with downy mildew is recorded by
examining a minimum of 25 plants each in two rows in different parts of each
field.

Data Collection, Interpretation, and Analysis

For the purpose of this paper, prevalence is defined
as the percent of fields with downy mildew and incidence is defined as the
percentage of plants infected in each field (15). Incidence is categorized into
three "yield impact categories": minimal (1-10%), low (10-20%), and moderate
(>20%). Although individual plants infected with downy mildew frequently
die and/or do not produce any yield (4,10), no literature on the relationship
between incidence of downy mildew in a field and yield loss could be found. As
such, studies relating stand reduction to yield were used to determine yield
impact categories. According to data compiled for crop insurance adjustors,
which assumes that random plant loss within seven weeks post-emergence, stand
reductions of less than 10% result in no yield loss, 10 to 20% stand reduction
results in a 1 to 3% yield loss and stand reductions of 30, 40, and 50% result
in an 8, 11, and 12% yield loss (1).

Rainfall data between 15 May and 15 June, when 90% of
the North Dakota sunflower crop is planted (2), was collected by North Dakota Agriculture
Weather Network (NDAWN,
ndawn.ndsu.nodak.edu) weather
stations each year of the survey. Pearson Correlation coefficients for rainfall
and disease incidence were calculated in SAS v 9.2 (SAS Institute Inc., Cary,
NC) using the PROC CORR procedure for 11 years of NSA and NDSU survey data to
analyze potential relationships between downy mildew and rainfall.

Mid-season Survey Results

A total of 1,558 sunflower fields were scouted
in the eleven-year survey, with a high of 205 fields in 2008 and a low of 79 in
2001 (Table 1, Fig. 4). Prevalence of downy mildew averaged 33.9%, with a range
from 1.6% to 70.3% in 2006 and 2011, respectively (Table 1, Fig. 5). The mean
incidence was 2.1%, ranging from 0.02% in 2006 to 7.1% in 2011. In each year,
the majority of fields infected with downy mildew had incidence values less than
10% and thus were categorized as having minimal yield impact. In 2005 and 2011,
the percentage of the fields infected with downy mildew had greater than 10%
incidence and were categorized as having moderate yield impact were 21.3 and
23%, respectively.

Fig. 4. Location of 1,558 sunflower fields in North Dakota surveyed by the mid-season survey between 2001 and 2011.

Fig. 5. Prevalence of sunflower fields with downy mildew as detected by mid-season and late-season surveys and total rainfall between 15 May and 15 June from 2001 to 2011.

Table 1. Number of fields, prevalence and incidence of
downy mildew between 2001 and 2011 as identified by the mid-season survey.

Year

Number
of fields surveyed

Prevalence of infected
fields

Percent fields with trace-10% incidence

Percent fields with 11-20% incidence

Percent fields with >20% incidence

Mean incidence across all fields

2001

79

32.9

29.1

2.5

1.3

1.5

2002

197

16.2

14.7

1.0

0.5

0.6

2003

123

20.3

18.7

0.8

0.8

0.7

2004

164

58.5

56.1

1.8

0.6

1.6

2005

80

58.8

37.5

12.5

8.8

5.9

2006

128

1.6

1.6

0.0

0.0

0.02

2007

130

21.5

19.2

0.8

1.5

1.3

2008

205

19.1

16.6

2.0

0.5

1.3

2009

182

48.4

42.9

4.9

0.5

2.6

2010

152

25.7

24.3

0.0

1.3

0.9

2011

118

70.3

47.4

11.0

12.0

7.1

Mean

141

33.9

28.0

3.4

2.5

2.1

Late-season Survey Results

A total of 1,214 fields were surveyed from 2001 to
2011 (survey not done in 2004), with a high of 266 fields in 2001 and a low of
77 in 2007, 2008, and 2011 (Table 2, Fig. 6). The mean prevalence of downy
mildew was 17%, with a low of 1.5% in 2001 and a high of 47.5% in 2005 (Fig. 5).
The mean incidence was 1.2%, with a low of 0.01% in 2001 and a high of 4.5% in
2005 (Table 2). The proportion of infected fields with a moderate yield impact
was high in 2005 and 2008, where disease incidence values were 26.3% and, 24.8%,
respectively.

Fig. 6. Location of 1,214 sunflower fields in North Dakota surveyed by the late-season survey between 2001 and 2011.

Table 2. Number of fields, prevalence and incidence of
downy mildew between 2001 and 2011 as identified by the late-season survey.

Year

Number
of fields surveyed

Prevalence of infected fields

Percent fields with trace-10% incidence

Percent fields with 11-20% incidence

Percent fields with >20% incidence

Mean incidence across all fields

2001

266

1.5

1.1

0.4

0.0

0.01

2002

252

4.0

3.6

0.0

0.4

0.5

2003

118

9.3

6.8

1.7

0.8

0.7

2004

na

na

na

na

na

na

2005

80

47.5

35.0

9.0

3.6

4.5

2006

84

2.4

2.4

0.0

0.0

0.08

2007

77

22.1

20.8

1.3

0.0

0.7

2008

77

20.8

15.6

2.6

2.6

1.6

2009

87

12.6

11.5

0.0

1.1

0.5

2010

96

9.4

9.4

0.0

0.0

0.5

2011

77

40.2

33.7

3.9

2.6

2.9

Mean

121.4

17.0

14.0

1.9

1.1

1.2

Comparison of Survey Techniques and Results

Although the survey methods varied in the number of
plants examined and number of locations surveyed per field, comparisons among
the two timings were made. The greatest difference between the mid-season and
late-season surveys was the time at which the survey is conducted. The
mid-season survey examined sunflower fields from late June until mid August,
which correlates approximately with the end of the vegetative growth stages
(approximately V8 to V12) to the beginning of the bloom stage (R5) (20), whereas
the late-season survey was conducted near physiological maturity. As neither
survey was designed specifically for downy mildew, both surveys may
underestimate the prevalence and incidence of downy mildew, because severely
infected plants may die prior to fields being surveyed. However, the mid-season
survey has a greater likelihood of more accurately assessing early infection, as
fields are scouted when plants that have died may still be visible.

Downy mildew prevalence results from the mid-season
survey were approximately twice as high as the late-season survey, although year
to year variation existed. Disease levels from 2004 were omitted, as the
late-season survey was not conducted that year. Data from both surveys
identified 2005 and 2011 as the years with high prevalence. However, surveys
disagreed on years with the lowest prevalence. Similarly, results from the
mid-season survey generally reported higher incidence and higher percentages of
fields where incidence exceeded 10%. Mean incidence values in the mid-season
survey (2.1%) were approximately double the mean incidence values in the
late-season survey (1.2%). However, both surveys identified 2005, 2008, and 2011
as having the highest proportion of downy mildew infected fields with incidence
values exceeding 10%. Data from both surveys suggest that yield loss due to
downy mildew was most significant in 2011, followed by 2005 and 2008.

Geographic Distribution of Downy Mildew

Downy mildew was observed in 51 of 53 counties in North Dakota
between 2001 and 2011. The disease was not observed in Divide and Williams
counties. Sunflower in those counties is limited and only nine total fields were
surveyed. Distribution of the most severely affected fields did not appear to
follow any geographic pattern and fields with > 10% incidence were scattered
throughout the state (Fig. 7).

Fig. 7. Location of 92 sunflower fields surveyed between 2001 and 2011 with downy mildew incidence greater than 10% as detected by the mid-season survey (triangles) and the late-season survey (circles).

Impact of Rainfall on Downy Mildew

Statewide rainfall totals were not significantly
correlated (P ≤ 0.05) to disease incidence for either of the surveys
(data not presented); which does not support the assumption widely held that
downy mildew is higher when it rains during the general planting time
(approximately 15 May to 15 June), and is only a limited problem when it is
relatively dry. Because saturated soil for only a brief time period immediately
after planting is needed for infection, brief localized thunderstorms and/or
field topography creating wet spots can result in significant infection even in
a relatively dry spring. Conversely, a dry period immediately after planting may
limit the development of downy mildew in a wet spring. It is likely that
correlations may be drawn between rainfall and downy mildew if specific planting
dates and rainfall history of each field is known, but this was beyond the scope
of this study.

Management of Downy Mildew: Past, Present and Future

Downy mildew has been a frequent problem of sunflower
in North Dakota, but levels of the disease fluctuates annually. Although micro-environment
may be the primary reason levels fluctuate, the fluctuation may be partially
related to the disease management tools available to growers. In the early
1980s, resistant hybrids were available, but new races of the pathogen
continued to evolve (10) and disease prevalence ranged from 23% in 1981 to 59% in
1984 (5). After the federal registration of the fungicide seed treatment metalaxyl (Apron, Syngenta Crop Protection) in 1985, downy mildew incidence
declined and seed companies relied more on chemical control rather than
developing resistant hybrids. A late-season survey conducted in 1991 found downy
mildew prevalence and incidence to be 16% and 0.05%, respectively, and a 1995
survey found prevalence and incidence to be 18% and 0.5%, respectively (5). In
the late 1990s, resistance to FRAC 4 fungicides developed in P. halstedii
populations and by 1999, 91% of the P. halstedii isolates tested in North
Dakota
were insensitive to the registered rates of either metalaxyl or mefenoxam (Apron
XL, Syngenta Crop Protection) (6,8).

From the late 1990s to 2002, neither effective seed
treatments nor resistant hybrids were available to growers. However, downy
mildew levels were relatively low during this period. In 2003, the fungicide
azoxystrobin was registered as a sunflower seed treatment and hybrids with the
Pl6 resistance gene became available, providing growers with two new
management tools (L. Kleingartner, personal communication). During this
period, downy mildew prevalence and incidence fluctuated from very high (2005)
to very low (2006), and remained relatively low until 2011. This may indicate
that the availability of new management techniques in 2003 did not immediately
alter the occurrence of the disease in the state. However, it is possible that
widespread prevalence and high incidence in 2005 resulted in greater adoption of
resistant hybrids and fungicide seed treatments in 2006 and in subsequent years,
resulting in lower disease levels. This could partly explain why high levels of
rainfall in 2007 and 2008 did not correlate with high disease pressure.

In 2009, P. halstedii races virulent on Pl6 were identified in multiple locations in
North Dakota (9), leaving FRAC 11 fungicide
treatments, which now included azoxystrobin and fenamidone (17), as the primary
management tool for the pathogen. In 2011, the prevalence and incidence of downy
mildew increased dramatically over recent years, which may have been a partly a
result of new pathogen races, which comprised 14% of isolates characterized
between 2009 and 2011 (9).

Future management of downy mildew will depend on
staying one step ahead of pathogen variability, which includes the deployment of
new resistance genes into commercial hybrids and the registration of effective
fungicide seed treatments that are not FRAC 11 or FRAC 4. Resistance genes
effective against the new P. halstedii races that confer virulence on Pl6
are in a limited number of commercial hybrids, but are available in released
USDA germplasm (9). However, the identification of new resistance genes is
important. Virulence changes in the pathogen occur, and it is likely that the
currently effective resistance genes will be overcome in the near future. FRAC
11 seed treatments are currently still effective, but the risk for the
development of pathogen resistance to this chemistry is high (17). As such,
effective fungicide seed treatments of additional FRAC groups will be important
for management of the disease in the future.

Conclusions

Downy mildew is an important sunflower disease in North Dakota,
but annual fluctuations in prevalence and incidence occur. These fluctuations
could not be easily correlated with statewide rainfall patterns at the time of
planting or management tools available to growers. However, monitoring
prevalence and incidence remains an important tool for assessing disease
pressure and may indicate possible changes in pathogen virulence or sensitivity
to fungicides. In this study, assessment of downy mildew in a mid-season survey
identified greater prevalence and incidence than in a late-season survey,
indicating mid-season assessment may be a more effective way to monitor downy
mildew levels in the future.

Acknowledgments

The authors thank the many participants in both
surveys, including surveyors and survey coordinators. The authors also thank the
National Sunflower Association, the North Dakota State Cooperative Extension
Service, and the United States Department of Agriculture – Agriculture Research
Service for logistical support and/or funding, and Drs. Michael Wunsch and Jared
LeBoldus for critical review of the manuscript.